The practical use of organic light emitting devices in display devices, light sources, illumination, etc., is being studied. In an organic electroluminescent element, an organic thin film is provided between a cathode and an anode; a voltage is applied between the cathode and the anode; excitons are created; and the light emitted when the excitons undergo radiative deactivation is utilized. Materials having relatively low conductivities such as, for example, ITO (Indium Tin Oxide) are used as the anode.
In the case where an organic electroluminescent element is applied in large surface-area illumination and the like, problems are expected because the conductivity of the anode is low, a voltage drop may occur in the plane, and the brightness may become nonuniform.
Moreover, to increase the luminous efficacy, it is important to efficiently extract the light emitted in the organic light-emitting layer.
JP-A 2006-156400 (Kokai) discusses technology to increase the outcoupling efficiency of an organic electroluminescent element by providing a diffraction grating layer. However, in such a method, it is necessary to form a fine diffraction grating. Therefore, it is difficult to practically apply such a method in an illumination device having a large surface area.
Special technology is necessary to increase the outcoupling efficiency while suppressing the voltage drop in the plane to practically use an organic electroluminescent element in an illumination device having a large surface area.